bes: The frontal lobe, which includes the front part of the hemisphere
and extends back to the fissure of Rolando and down to the fissure of
Sylvius; the parietal lobe, which lies back of the fissure of Rolando
and above that of Sylvius and extends back to the occipital lobe; the
occipital lobe, which includes the extreme rear portion of the
hemisphere; and the temporal lobe, which lies below the fissure of
Sylvius and extends back to the occipital lobe.

THE CORTEX.--The gray matter of the hemispheres, unlike that of the
cord, lies on the surface. This gray exterior portion of the cerebrum is
called the _cortex_, and varies from one-twelfth to one-eighth of an
inch in thickness. The cortex is the seat of all consciousness and of
the control of voluntary movement.

[Illustration: FIG. 10.--Different aspects of sections of the spinal
cord and of the roots of the spinal nerves from the cervical region: 1,
different views of anterior median fissure; 2, posterior fissure; 3,
anterior lateral depression for anterior roots; 4, posterior lateral
depression for posterior roots; 5 and 6, anterior and posterior roots,
respectively; 7, complete spinal nerve, formed by the union of the
anterior and posterior roots.]

THE SPINAL CORD.--The spinal cord proceeds from the base of the brain
downward about eighteen inches through a canal provided for it in the
vertebrae of the spinal column. It is composed of white matter on the
outside, and gray matter within. A deep fissure on the anterior side and
another on the posterior cleave the cord nearly in twain, resembling the
brain in this particular. The gray matter on the interior is in the form
of two crescents connected by a narrow bar.

The _peripheral_ nervous system consists of thirty-one pairs of
_nerves_, with their end-organs, branching off from the cord, and twelve
pairs that have their roots in the brain. Branches of these forty-three
pairs of nerves reach to every part of the periphery of the body and to
all the internal organs.

[Illustration: FIG. 11.--The projection fibers of the brain. I-IX, the
first nine pairs of cranial nerves.]

It will help in understanding the peripheral system to remember that a
_nerve_ consists of a bundle of neurone fibers each wrapped in its
medullary sheath and sheath of Schwann. Around this bundle of neurones,
that is around the nerve, is still another wrapping, silvery-white,
called the neurilemma. The number of fibers going to make up a n